1. Field of the Invention
The invention relates in general to a memory and a method of fabricating the same, and more particularly to a memory having two metal oxide layers and a method of fabricating the same.
2. Description of the Related Art
Integrated circuits are used to control the functions of many modern electronic devices. Memory devices used for storing (write) and retrieving (read) data possess advantages that are easily accessible by the integrated circuits. Various types of memory devices for data storage are known in the art. Generally, memory devices are distinguished by their speed and data retention characteristic.
Memory device falls mainly into two classes: random access memory (“RAM”) and read-only memory (“ROM”). Many improvements and variations of RAMs and ROMs have been advanced to further their performance, and both types of memory have their own advantages and disadvantages. Typically, RAM (i.e. volatile) has fast data transfer rates and efficient writing architectures, but requires continuously power to keep the data. ROM retains the contents even in the absence of power, but is limited in the speed, the number of times to which it may be written, and the manner in which it may be written. ROM mostly comprises Flash memory, EPROM, OTP, EEPROM, and PROM. RAM mostly comprises static RAM (SRAM), dynamic RAM (DRAM).
Static random access memory (SRAM), with very fast access times, retains a value as long as power is supplied. However, its volatility (i.e. it will lose its contents when the power is switched off), large size and stand-by current limit the total size and applications of the memory. DRAM has the smallest cell size, but necessitates a complex refresh algorithm, and is also necessary to supply voltage at all times or the information will disappear (i.e. volatile). Flash memory (i.e. non-volatile memory) is slower to program, and in some cases must erase a large block of memory before being reprogrammed.
Resistive random access memory (RRAM) is a new non-volatile memory type being developed by many companies for a period of time. Different forms of RRAM have been disclosed, based on different dielectric materials, spanning from perovskites to transition metal oxides to chalcogenides. Literature data are giving more indications that RRAM is closest to becoming a universal memory. For, example, RRAM operates at a faster timescale (switching time can be less than 10 ns), and has a simpler, smaller cell structure (a 4-8 F2 MIM stack). Compared to flash memory, a lower voltage is sufficient. Compared to DRAM, the data is retained longer (10 years).
Memory devices can be constructed into two-dimensional (2D) cell array or three-dimensional (3D) memory array, wherein the 2D memory cells are matrixed in the X-Y plane. The 3D memory device includes a plurality of stacked memory cells to form a three-dimensional memory array, and it can be applied to multi-level operation after properly designed. FIG. 1 shows a three-dimensional memory device capable of being applied to multi-level operation. Referring to FIG. 1, the three-dimensional memory device 10 includes three vertically stacked structures 21, 22, and 23. Each of the stacked structures includes electrodes 11, plug 12 and metal oxide layer 13 and fabricated by COA/VIA1/VIA2 process sequentially. Accordingly, more steps and cost is needed to fabricate the 3D memory device 10. More risks caused by complicated steps are also introduced in fabrication.